Quantum "vs" Classical ?

[CraigS]

Is there really a fundamental difference between a quantum mechanical system and a classical physics system ?

When someone says that a system is 'fundamentally a quantum system', does this mean that it is somehow different from a classical system ?

If so, what is the fundamental difference ?

Cheers

[xelasnave]

Still looking at this and have yet to form an opinion.

alex

[avandonk]

When the mathematical functions describing the system are no longer piecewise continuous.

There is another region when functions asymtote to infinity.

Apart from that simple linear equations will get you by if you believe in a simple Universe! If you really want to be a smartarse you can invoke higher order polynomials.

Bert

[traveller (Bo)]

If you haven't read it, I highly recommend StepheN Hawkings a brief history of time. Even I managed to understand most of the concepts there

[CraigS]

Thanks Bert and Bo;

Very interesting … I really don't know the answer to this question myself, so I really appreciate everyone's views on it.

It just seems to me that it is frequently said that a system is 'fundamentally behaving in a quantum mechanical way', because we cannot understand it with a classical model.

But perhaps QM kind of soaked up what might have been missing from the assumptions about single particle behaviours, which were kind of 'missed out' when classical mechanics was developed ?

There seems to be an inordinate amount of time and energy expended trying to "unite the two worlds" into a common 'framework' and yet, it kind of occurs that maybe the 'gap' (or discontinuities) which everyone is focussing on, may actually simply be more about, (for want of a better term), 'finger-pointing', aimed at highlighting the errors originally made by expecting single particles to behave in ways, they basically never did in the first place … and, perhaps it is simply 'expectations' that are in error.

For example, the double slit experiments seem to end up being described as 'fundamentally quantum mechanical', because the interference patterns can't be explained when we 'imagine' the way we think a single particle could possibly behave. Perhaps this is purely because our 'imaginings' about how particles behave, simply weren't accurate in the first place, and we were missing something fundamental in the classical descriptions of how particles in nature really behave (??)

Is any of this saying anything fundamental about nature/the system, or is it saying something about our models ?

Are there any physical behaviours of a quantum system which don't depend on an expectation of how we think classical particles behave ?

Cheers

[sjastro]

A classical physics system doesn't provide the nuts and bolts description of a quantum mechanical system.

Classical physics primarily involves the use of phenomenological theories.
By phenomenological it means that classical physics is built on explaining observation and experimental data without necessarily providing a mechanism.

For example the apple falling from a tree is explained by gravity, yet Newton's theory of gravitation provides no clues as to what causes gravity.

A classical physics system is an approximation of a quantum mechanical system.

Physicists empirically determined the Balmer series for the spectral lines of the hydrogen spectrum in the 19th century and explained it as atomic emissions.
This is classical phenomenological physics at work.

Quantum mechanics developed in the early 20th century on the other hand provided the explanation for the atomic emissions.

Regards

Steven

[CraigS]

Hi Steven;

Thanks for your post.
Yes, I agree … its all very interesting, and the differences between the two approaches is more often than not, portrayed in terms of how QM evolved in history, as primarily a challenge to classical physics.

As far as the historical significance is concerned, perhaps the development of QM serves more as a kind of reminder that the phenomenological approach can, and does, leave gaps in the explanations and those gaps are likely to be filled by someone ... given an 'alternate name', and forever after will be seen as representing something fundamentally different in nature ?

Whilst I think following the historical development of QM might help in learning about QM, I’m not so sure this approach necessarily aids in re-inforcing the accuracy of QM, in so far as it more closely representing reality (whatever ‘reality’ is, eh ... might leave that one aside for the moment ). Sometimes the QM arguments are used in this sense over classically based arguments. I suppose at the end of the day, QMs evidence based, empirical track record speaks more directly to this point.

However, I wonder what might have happened if incomplete classical physics, simply proceeded on its normal course .. would it have eventually encompassed everything QM does, but in its own descriptive language and the ‘gaps’ we now see wouldn’t appear, and lead us to the conclusion that there seems to be a fundamental discontinuity in nature (along the lines suggested by the mathematical approach .. as in Bert’s post) ?

Steven mentioned the Balmer Series for hydrogen. Is it now possible to generate a discrete spectrum using classical wave principles (I wonder) ?

Cheers

[sjastro]

Originally Posted by CraigS

Hi Steven;

Thanks for your post.
Yes, I agree … its all very interesting, and the differences between the two approaches is more often than not, portrayed in terms of how QM evolved in history, as primarily a challenge to classical physics.

There is no "conflict" between the two theories. QM is an evolution of classical physics. It hasn't replaced classical physics but takes it to a more fundamental level.
The slit experiment is explained classically by treating light as a wave. QM extends this to wave/particle duality.

Steven mentioned the Balmer Series for hydrogen. Is it now possible to generate a discrete spectrum using classical wave principles (I wonder) ?

Bohr came up with a "semi classical" or "classical/QM hybrid" which explained the spectral lines. This was however part of the evolution of classical physics to QM.

Regards

Steven

[CraigS]

Originally Posted by sjastro

There is no "conflict" between the two theories. QM is an evolution of classical physics. It has hasn't replaced classical physics but takes it to a more fundamental level.

Yes .. again, I can whole heartedly see this also.
(A first for CraigS, eh ? … woo hoo ! …)
.. However, I'm afraid quite a few others don't, though.

Originally Posted by sjastro

The slit experiment is explained classically by treating light as a wave. QM extends this to wave/particle duality.

So then, perhaps the discontinuities made obvious when we compare 'the fundamentally quantum system' with the 'fundamentally classical system', were always there in the classical model, but were unknown and unrecognised ?… And when QM came along, it did expose the discontinuities .. but these were always present because of the ways Classical was attempting to explain wave and particle dynamics ?

I know its a subtle difference, but I hope others might see that there's been a huge amount of effort expended to unify 'two different worlds' by asking very deep questions like: "why the behaviours of the big and the small don't match up ?", when it could simply have appeared this way because the descriptions of wave and particle dynamics in classical physics were incomplete somehow ?
(I think we're on the same tracks here, also).

Originally Posted by sjastro

Bohr came up with a "semi classical" or "classical/QM hybrid" which explained the spectral lines. This was however part of the evolution of classical physics to QM.

Fair enough, too … and its interesting to note that we still refer to the differences, (discontinuities really), as somehow a function of some different behaviours, encompassed by the labels: "QM" and "Classical".

I mean, how often is "QM behaviour" referred to as "spooky" or "weird", seemingly to evoke an alluring perception of something which QM provides us with over classical ? (This is a perfect breeding ground for nutters who for example, hijack QM terminologies to suit their own malicious intents).

Cheers

[xelasnave]

It is difficult to form an opinion on two matters one knows very little about and that is indeed part of my difficulty however the ideas outlined in this thread I have found more helpful in moving forward than any other material I have read. I thank you all.

This thread identified an undefined concern I have had for some time.

I find Steven's statement to be most helpful.

...There is no "conflict" between the two theories. QM is an evolution of classical physics. It has hasn't replaced classical physics but takes it to a more fundamental level.

I have felt that GR had closed the door on gravity (in so far as it is seemed to me to be presented that way) and whilst being presented with a feeling such to be reality I felt gravity must have a mechanical explanation.

I now dont feel guilty to suggest that possibly GR maps the activity of something smaller and that the fabric of space may be woven from particles or quanta that gives it a tangibility that seems to confilct with the fundamentals of GR. I never saw there need be a conflict.



alex

[sjastro]

Originally Posted by xelasnave

It is difficult to form an opinion on two matters one knows very little about and that is indeed part of my difficulty however the ideas outlined in this thread I have found more helpful in moving forward than any other material I have read. I thank you all.

This thread identified an undefined concern I have had for some time.

I find Steven's statement to be most helpful.

...There is no "conflict" between the two theories. QM is an evolution of classical physics. It has hasn't replaced classical physics but takes it to a more fundamental level.

I have felt that GR had closed the door on gravity (in so far as it is seemed to me to be presented that way) and whilst being presented with a feeling such to be reality I felt gravity must have a mechanical explanation.

I now dont feel guilty to suggest that possibly GR maps the activity of something smaller and that the fabric of space may be woven from particles or quanta that gives it a tangibility that seems to confilct with the fundamentals of GR. I never saw there need be a conflict.

alex

First of all my comment.

...There is no "conflict" between the two theories. QM is an evolution of classical physics. It has hasn't replaced classical physics but takes it to a more fundamental level.

Remove the "has" term and it makes sense, "has hasn't" implies quantum duality in itself. It's what happens when your brain is in QM mode.

GR is a phenomenological theory so it doesn't explain the mechanism of gravity despite Brian Cox in a recent SBS documentary suggesting otherwise.

Regards

Steven

[CraigS]

Originally Posted by sjastro

Remove the "has" term and it makes sense, "has hasn't" implies quantum duality in itself. It's what happens when your brain is in QM mode.

Hmm … I didn't even notice this .. and if I had, my brain would have seen it as a typo !
I'm left wondering about the explanation, however …
(This is fun .. and pretty 'geekish' ..).

Originally Posted by sjastro

GR is a phenomenological theory so it doesn't explain the mechanism of gravity despite Brian Cox in a recent SBS documentary suggesting otherwise.

Hmm .. once again, it seems that the label "phenomenological theory" could also be interpreted as a way of confining Classical .. and could also be seen to preserve what we've established as only a perception/historically evolved matter.

I think the language is a big culprit in creating this perception.

Very interesting .. and thank you to all, for such an enlightening discussion!

Cheers

[renormalised (Carl)]

Remove the "has" term and it makes sense, "has hasn't" implies quantum duality in itself. It's what happens when your brain is in QM mode.

Just goes to show you, Steven, that's how the brain works....it's quantum

Guys...to put it in plain and simple terms, what Steven is saying, is that QM is an "extension" of Classical Physics. It takes what is already known and extends it to a new level of understanding. Instead of just explaining what is happening, it is looking at why it happens as well. Instead of just looking at the forest, it's also looking at how the trees create the forest.

[xelasnave]

Thanks Steven

mmmm Interesting I interpreted things to suit myself... curious.

I think words fit their context... but it seems we both knew what you were saying.

alex

[xelasnave]

Originally Posted by renormalised

Just goes to show you, Steven, that's how the brain works....it's quantum

Guys...to put it in plain and simple terms, what Steven is saying, is that QM is an "extension" of Classical Physics. It takes what is already known and extends it to a new level of understanding. Instead of just explaining what is happening, it is looking at why it happens as well. Instead of just looking at the forest, it's also looking at how the trees create the forest.

and String theory is looking at the leaves?

I like the way you put that Carl.


alex

[CraigS]

Originally Posted by xelasnave

and String theory is looking at the leaves?

String theory, I think, has also evolved to bridge the gaps, (which have always existed), in the way Classical attempts to explain wave and particle dynamics.

By replacing particles and waves by another meta-'object', a string, an integrated view emerged. Only problem is that the unexplained particle and wave 'objects' remain (over to 'QM' to sort this out). I think the string theorists recognised the integrating power of creating a new view .. but this doesn't necessarily mean that the fundamental objects in the universe are strings.

String theory, I think, carries another message for us .. which is not necessarily related to the reality of nature.

Cheers

[xelasnave]

My impression is a string is a way of visualizing (quantifying etc) the movement of a particle which can be shown as a wave function...and in that form it is something we can work with... I dont know if that is valid but that is where I am up to Craig.

I was comptemplating the energy available at a point in space coming from a place where I would use only the smallest quanta of energy..what ever that may be...energy at a point would be probable as the number of trajectoies less a number that did not convey energy times the smallest quanta...this is my approach out of respect for maths Craig..using only the little I have to work with...anyways a goggle turned up some fella who had worked out the smallest quantum (by his system) only to go on to expound the push LeSage approach and tie it in with GR.. new theory of everything..he had the maths etc but the net has let me off pursuit of the matter because as with most things not only has it been done but done at a level one never could have reached... but I had to laugh.. the guy may be a crack pot..that is not the issue really... so its been done to death...and he has a book

http://www.rostra.dk/louis/

Does string theory treat the photon as a fundamental particle??? must check...and what is a fundamental particle really made from...the russian doll spiral of my imagination knows no bounds.

alex

[bojan]

Originally Posted by sjastro

By phenomenological it means that classical physics is built on explaining observation and experimental data without necessarily providing a mechanism.

For example the apple falling from a tree is explained by gravity, yet Newton's theory of gravitation provides no clues as to what causes gravity.

Yes.. phenomenological... or, maybe the better term might be behavioural..

However, QM - while cutting much, much deeper into mechanisms of nature - still doesn't provide explanation as to what causes gravity, or electrical charge, what indeed is elementary particle... and why they are elementary (IF they are).
So, we can say that even QM is an approximation of "higher level" physics (strings? or something entirely different..), without conflicts again of course.

[sjastro]

Originally Posted by bojan

Yes.. phenomenological... or, maybe the better term might be behavioural..

However, QM - while cutting much, much deeper into mechanisms of nature - still doesn't provide explanation as to what causes gravity, or electrical charge, what indeed is elementary particle... and why they are elementary (IF they are).
So, we can say that even QM is an approximation of "higher level" physics (strings? or something entirely different..), without conflicts again of course.

All theories are phenomenological to various degrees and can change according to the physical processes they describe.

For example GR is phenomenological with respect to gravity, it treats gravity as an example of Newton's Third Law in operation.
With regards to the effect of gravity on light, GR is non phenomenological. The gravitational bending of light was predicted by the theory, rather than observation used to formulate the theory.

Regards

Steven

[CraigS]

I think that even though String Theory may not have started out with the goal of unification, (which it is now recognised as providing), the lesson here may be one of how to go about unification in theoretical physics.

QM also seems to have unified other problem parts of classical physics like the gaps between particles and fields, and a favourite of mine: deterministic and random behaviours (which doesn't appear to have been really happening prior to its conception).

The problem seems to be that it was seen as a competing idea. Nowadays, the driving 'trend' is towards unification (of just about everything) and String Theory is more 'acceptable', and is hence met with more open arms.

I guess it could also be said that QM came about because experimental/observational science got way ahead of theory, too?

Interestingly though, String Theory seems to be around the other way!

Cheers